I built a digging box for my pet rats a few years ago, using a ten
gallon aquarium filled two thirds of the way up with potting soil. I
placed my rats, Paint and Smudge, in there and they loved it. They
started digging in one corner and eagerly dug a deep (6 inch) hole in
one corner right down to the bottom of the aquarium. However, they
couldn't dig a tunnel, because the dirt was too soft and crumbly.

Therefore, I've been on the lookout for a description of larger
setup, something that would allow serious burrow and tunnel digging.
I found just such a giant digging box in an article by Boice (1977).
Boice studied digging behavior in wild and domestic rats. For this
purpose, he constructed a glass-sided digging box, and tried many
different soil combinations to find the one that worked best. Here
are Boice's specifications:

Overall appearance

This is a wide, tall, shallow wooden box with glass sides. The box
is large enough to allow the rats to dig tunnels and nest chambers,
and the glass sides allow you to see the rats underground. It is
filled with a special mixture of soils that (1) encourages rats to
dig, (2) is strong enough to support tunnel walls, and (3) is moist
enough to prevent excessive drying.

Box dimensions: The box was made of soft pine lumber. It
measured 112 cm wide x 60 cm tall x 36 cm deep. There were glass
panes on the two largest sides measuring 100 cm wide x 50 cm tall to
permit underground viewing. Exposed wood was covered with fine (0.25
in) wire mesh to prevent chewing, and the whole chamber had a 0.5
inch hardware cloth top with a hole for a water bottle.

Soil mixture: After much experimentation, Boice found that
a mixture of 1/3 humus soil, 1/3 clay soil, and 1/3 sand encouraged
burrowing but prevented the collapse or excessive drying of the
tunnel walls.

Moisture: In the laboratory where this experiment was
conducted, the ambient temperature was 20º C and the humidity
50%. To keep the dirt from getting dry, the dirt was dampened every
other day with 1/6 of a gallon of water from a garden sprinkling can.
The water was sprinkled evenly and slowly over the dirt.

Digging stone: To get the rats started, it was critical for
them to be provided with a "digging stone," because rats prefer to
begin digging under objects. Boice used a concrete patio stone (43 x
19 x 4 cm) placed on top of the dirt for this purpose.

Cleaning: Boice found that the surface dirt tended to
solidify and get fouled by the rat. To keep the dirt clean, this
surface dirt (about 20% of the total) was replaced every10 days. All
the dirt was replaced every 30 days.

Length of stay: The digging box was not intended as
permanent housing for rats. In this study, each rat stayed in the box
for 10 days. When the 10 days were up, Boice removed the rat to a
cage and studied the burrows. Then he replaced, smoothed and
moistened the dirt (surface soil was replaced every 10 days, all soil
every 30 days), and introduced a new rat into the digging box.

What did Boice's rats do in the digging box?

All rats placed in the digging box dug tunnels and burrow systems,
except for older males. Most burrow digging occured in the first few
days. All burrows started under the digging stone. Burrows typically
consisted of an entrance tunnel followed by a nest chamber. Some
burrow systems grew quite elaborate and had tunnels and chambers on
more than one level, and some circled back to reconnect.

Figure 2. Drawing of a
typical burrow system. The tunnel starts beneath the digging
stone. The nest chamber is at the end of the first tunnel
segment. A second tunnel wraps around the front of the box.
If continued, it would probably have turned up into a bolt
hole. Adapted from Boice 1977.